<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box{
            width: 100px;
            height: 100px;
            border: 1px solid #000;
            display: inline-block;
        }
    </style>
</head>
<body>
    <div>
        <button>red</button>
        <button>yellow</button>
        <button>blue</button>
        <button>green</button>
    </div>
    <br>
    <div class="box"></div>
    <div class="box"></div>
    <div class="box"></div>
    <div class="box"></div>

    <script src="../js/jquery.js"></script>
    <script>
        let color = null;

        $('button').click(function(){
            color = $(this).html();
        })

        $('.box').click(function(){
            $(this).css('background',color);
        })
    </script>
</body>
</html>